The 3dB bandwidth spec may not be such a big deal. As a QRO fan of homebrew plate modulation and HiFi... let me explain: When the FCC (or anyone for that matter) specifies a bandwidth, the bandwidth attenuation by definition are the -3 dB points, usually with no mention of any other attenuation levels, or more specifically the "shape" of the response mentioned (other that -3dB). To be "in spec" one must be 3dB down at 3 kHz (not to noticable), however as an example, the 4 kHz point could be only -4dB!!! and still be OK since the skirts of the response are not specified. If a station receives your signal at +/- 5 kHz, he would have a hard time "proving" that the 3Khz point is not -3dB, especially if the signal is at +20 dB over 9 at the center of his dial! At this received signal strength and proposed -3dB points... the received signal of 3 kHz audio would be at +17 DB over 9 (-3dB)assuming at 10kHz bandwidth receiver. Even if the roll-off of the transmiiter provided -30 dB attenuation at 8 kHz, the receiver would still see an S-7 signal at +/- 8Khz! All this also assumes the receiver's -3db points are ignored when concidering the "complaintent". Since most modern SSB receivers have very steep skirts, the "hifi" off channel received signal would be attenuated a great deal. Of course... the FCC measuring equipment is the spectrum analyzer with the inherintly flat broadband receiving display. Even using an analyzer... 3 kHz at -3db down is not an easy measurment to make (using voice)unless the transmitter is frequency swept while being monitored. I feel we AMers can proceed as we have been, using good engineering practices with no "additional resrictions". This is really not out of line with what "un-AMers" have been seeing in their receivers all along anyway. So... not to worry. Just my .02 cents, George AB2KC
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Donald Chester Sent: Monday, August 26, 2002 1:00 AM To: [email protected] Subject: [AMRadio] Specific Bandwidth Limits for AM Another point to consider... >From the looks of Rinaldo's letter, the ARRL may be considering petetioning the FCC for specific bandwidth limits of 3 kc for SSB and 6 kc for AM. I see a couple of problems with that idea, besides the fact that it would limit phone signals to telephone-like "communications quality" audio. Even the SSB'ers are getting away from that, with the current interest in "HI-FI" SSB (an oxymoron?) First, even if you wanted to limit the upper high frequencies to 3 kc, it would be IMPOSSIBLE to maintain a flat response that high, without transmitting a signal bandwidth of at least 4 kc for SSB and 8 kc for AM, due to the rolloff characteristics of even the best filters. There is no such thing as an audio or rf filter with vertical skirts. Even the famous Collins mechanical filters are rated at -3/-30 dB points. Usually the -30 dB point is nearly twice the -3 dB point. To limit the significant bandwidth of a phone signal to 3 or 6 kc would require the audio to start rolling off at 2 kc or less. Secondly, this would render most presently-used vintage AM rigs and many older SSB rigs obsolete. Most AM rigs simply use a mic driving an audio amplifier which in turn drives the modulator. There may be some bypass capacitors across an audio transformer or two, and/or across the plate resistors of some of the low level audio stages, but this generates, at best (or worse, depending on what you think of the idea), a gradual rolloff of highs. Extremely sharp audio filters are possible (I have one), but not generally available, and to install them would require what would undoubtedly be difficult and expensive modifications to existing rigs. If this becomes mandatory, I suspect that many AM'ers will simply give up and say hell with it. So maybe the bandwidth proposal would not immediately kill AM as Docket 20777 would have, but in the long run it would have the same effect. Hopefully, knowledge of what is presently going down is not limited to members the AM Reflector and those who monitor the AM Window BB. There are still many members of the AM community who don't have computers or internet access. Also, I think many in tho SSB community would be allied with us on this issue, if they were aware of what may be about to happen. Don K4KYV _________________________________________________________________ MSN Photos is the easiest way to share and print your photos: http://photos.msn.com/support/worldwide.aspx _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio
